I have just finished an oddball August Carlstedt big parlor with a whopping 67,5 cm scale length (mensur), 26.6" for you Americans. It's a light built guitar with wide sides and big volume in the smallish body.
I feel that the sustain is shorter on this guitar than expected and looked up how sustain is affected by scale length alone. There were two schools of thought. Longer and shorter!
Now I wonder, is there a truth out there? Longer? Shorter?
I have always thought that volume and sustain are opposite cousins - you have a certain amount of energy from the strings, you can have one but lose on the other, so achieving a balance is required. I have thought that too high a tension can have a detrimental effect on sustain as it increases volume, and sometimes a long scale length is an attempt to increase volume. You might try low tension strings and de-tuning a full step to D-D to see what happens.
I personally think it’s more to do with the instruments construction than the length of the scale.
Discovered that the short sustain mostly came from my arm resting on the top! Did som more digging on the "net" and this particular question seems to be unexplained and in need of some serious experiments from someone. Anyway. This is the guitar.
Nice looking guitar.